Boeing tweaks design of 737 MAX engine

The jet-maker has decided to increase the crucial engine fan diameter from 68 to 69 inches. The size of the fan on the new jet has been a matter of contention because Airbus is able to offer a much bigger fan on the coming update to its rival single-aisle family, the A320neo.

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Boeing is continuing to tweak the design of its coming 737 MAX, the update to its workhorse single-aisle jet due in 2017, and has decided to increase the crucial engine-fan diameter from 68 to 69 inches.

Boeing spokeswoman Lauren Penning said the change stemmed from wind-tunnel testing that was completed last week and continuing work with engine-maker CFM International, a joint venture between GE and Snecma of France.

The size of the fan on the new jet has been a matter of contention because Airbus is able to offer a much bigger fan on the coming update to its rival single-aisle family, the A320neo.

A bigger fan produces more efficient propulsion. On the negative side, it also adds weight and drag. Engineers must come up with the optimal engine size to produce the greatest overall benefit to the airplane.

Penning said 69 inches is "looking like the best balance of weight, drag and performance."

However, on fan size, Airbus insists that bigger is better. Its executives argue that Boeing simply can't make the 737 MAX fan as big as it would like because the jet sits lower to the ground than the Airbus A320 and there isn't enough clearance to fit a bigger fan.

On the MAX, Boeing is offering just one engine: CFM's LEAP-1B.

Airbus is offering a choice between a variant of CFM's LEAP with a 78-inch fan and a Pratt & Whitney geared engine with an 81-inch fan.

In April, Boeing announced a series of design changes to the MAX, including the 8-inch lift to the nose gear, a change in shape of the tail cone and the introduction of fly-by-wire spoilers on the wings. Then earlier this month, it revealed a new type of winglet for the MAX.

Some in the industry have speculated that, because of the ground-clearance limitation on fan size, Boeing is struggling to come up with a design that will match the fuel efficiency of the Airbus neo.

But in a note to clients Wednesday, Richard Safran, aerospace analyst with Buckingham Research Group, wrote that "the revised engine fan size has more to do with optimizing the engine than a means to overcome performance deficiencies."

And Scott Hamilton, industry analyst with Leeham.net, said that with the MAX still five years away from entry into service, "Boeing is doing what it ought to be doing in trying to get every little advantage out of its redesign."